The training platform in this proposal focuses on intracellular signaling in B lymphocytes as a new approach to correcting fundamental defects in immune tolerance and to monitoring therapeutic interventions to reprogram islet-destructive immunity. While much work has gone into understanding the intercellular processes that drive T1D, the intracellular cascades that control the destructive responses to islet antigens are undefined. The production of autoantibodies is the earliest available biomarker of autoimmunity, but the mechanisms that permit this breach in immune tolerance remain unknown. While targeting of the whole B lymphocyte pool with rituximab delayed the progression of new-onset T1D, the effect was transient, which may relate to failure to reprogram islet-reactive cells long-term. Like T cells, B cells progress through multiple developmental stages during which they are programmed by antigen contact to be tolerant to self-antigens. This program fails in T1D, which I have hypothesized is due to improper coupling between antigen contact and effector responses during B cell development. In my preliminary studies, I have utilized new approaches in cutting-edge, phosphoflow cytometry to uncover defects in c-Abl signaling in the Transitional 1 (T1) B lymphocyte subset of NOD mice, the benchmark preclinical model of T1D. While the function of c-Abl in oncogenesis has been extensively dissected, its role in the programming of immune tolerance in B cells is largely unknown. In this proposal I will dissect the c-Abl dependent signaling pathways that govern B lymphocyte tolerance mechanisms. I hypothesize that c-Abl modulates the balance between homeostatic expansion and negative selection in the transitional B lymphocyte compartment and that defects in this pathway underlie B lymphocyte selection defects found in autoimmune disease. I will dissect c-Abl mediated pathways that control B lymphocyte homeostasis and selection by applying transgenic, antigen-specific systems and therapeutic targeting of c-Abl pathways for disease modulation (Aim 1). I will extend this analysis of immune signaling to understand how immune function is reprogrammed during disease-modifying therapy using informatics approaches combined with high dimensional mass cytometry (Aim 2). Through this fellowship application, I will develop 1) a detailed understanding of new intracellular pathways of B cell regulation during tolerance induction and 2) my potential as a basic and translational scientist focused on T1D. These training goals will be facilitated by the research plan, by the exceptional team of mentors with expertise in the proposed study design, by completion of the Howard Hughes Certificate Program in Molecular Medicine, and by the resources, facilities, and training plan available through Vanderbilt University.